Secondary Glazing for Sash Windows

Three separate paths are at work. First, the glass itself: 4mm float has a critical frequency that coincides with traffic noise, so it re-radiates rather than blocks. Second, the perimeter gaps at the meeting rail, staff bead and parting bead, which typically total 200-400cm² of open area. Third, the pulley stile boxes, which connect the room directly to the outside via the weight cavity.

Replacement double glazing addresses only the first path, which is why homeowners who replace sashes for acoustic reasons are so often disappointed. Secondary glazing addresses all three at once by placing a sealed independent barrier across the whole reveal.

Performance is a product of three variables: glass mass, cavity depth and seal integrity. Skipping any one of them caps the result. Our standard high-performance sash specification is 10.8mm acoustic laminated glass (two panes bonded with an acoustic PVB interlayer), set at 150mm from the original sash, with compression EPDM seals on all closing edges.

The interlayer matters more than raw thickness. Acoustic PVB damps the coincidence dip that plain laminate suffers from, which is worth 4-5dB in exactly the frequency band where traffic and voices sit.

Vertical sliders mirror the movement of the original sash and are the intuitive choice on tall Georgian boxes. Horizontal sliders are lighter to operate and generally preferred on Victorian two-over-two sashes. Where shutters are in use, hinged units swing clear of the shutter box.

Because nothing is removed and all fixings go into the reveal lining rather than the original joinery, secondary glazing is the only glazing upgrade routinely approved on Grade I and Grade II listed sash windows. Historic England explicitly identifies it as a reversible, conservation-compatible intervention.